I hope you have not added any fluid except DOT 3 or DOT 4 brake fluid to the brake master cylinder.
Brake master cylinders have what is called a "residual pressure check valve" in the outlet gallery of the cylinder.
The purpose of this valve is to permit all but about 4 to 6 PSI of brake brake fluid pressure be returned to the master cylinder when the foot pedal is released. The remaining (residual) pressure is trapped in the brake circuit to keep the wheel cylinder rubber cup lips "inflated" and thus pressed tightly against the cylinder walls to minimize occurrence of leaks there.
It may also be possible for trash and sludge in the master cylinder bore to to have migrated to the residual check valve and interfere with it letting ANY pressure to relieve back through it.
If ever any petroleum based oil, or silicone based (DOT 5) brake fluid was poured into the brake master cylinder reservoir, the rubber residual pressure check valve has probably been ruined.